It is known that soil particles contain a large number of small channels or capillaries through which water is capable of flowing, and may be graded on the basis of the capillary or pore diameters. As water is made to flow through a channel, whether that channel be a soil pore or not, the rate of capillary water flow through the channel will be higher if the water is capable of wetting the channel surface. At the interface of the water and the capillary surface, however, there exists a long range van der Waal interaction between the water and the capillary surface. While the van der Waals interaction typically extends less than 200 angstroms into the body of water, it nonetheless decreases the ability of the water to wet the capillary surface, thereby increasing the contact angle between the water and the capillary surface and hindering the flow of water therethrough. While the negative effect of the van der Waals interaction may be negligible in the case of water flowing through a household pipe, when one considers the flow of water through minute soil pores, this interaction has a major effect.
Agronomists and farmers have to work with all types of plant growth media such as sand, natural earth, horticultural soils, and various soil-mimicking, soil-less plant culture substrates; however, the bane of essentially all agriculturalists is an hydrophobic/water repellent soil. Water repellent soil retards water infiltration into the soil matrix and often renders entire areas of the upper layers of the soil substrate essentially impervious to water penetration. Under rainfall or irrigation conditions, dire environmental consequences can result from the water repellency of the topsoil, such as surface runoff of water and aqueous compositions containing pesticides, this term inclusive of fertilizers, into pristine areas and/or potable reservoirs. Furthermore, and less obvious, are the serious consequences that result from aqueous pesticide flow through “fingers” that usually attend water repellent soil which can provide rapid transport of pesticide compositions to the local ground water table and thus increase the risk of ground water contamination.
The hydrophobicity/water repellency of a soil is not only a function of the initial water content of the soil, but is also a function of soil particle size (sands are more prone to water repellency than clays), as well as, type of organic matter incorporated in it. This organic matter induces water repellency in the soils in many ways, such as by providing hydrophobic organic substances leached from the plant litter; organic substances that have been irreversibly dried; and microbial by-products.
Before water will evenly infiltrate into or percolate through a soil matrix, there must be a continuous film of water on the soil particles. In other words, the soil must first be wetted before water will flow. In addition, getting the soil evenly wetted is of paramount importance to the healthy growth of plants or seeds which are to be grown in the soil. Thus, agriculturalists will often apply various wetting agent surfactant compositions directly to the soil.
Although an increasing number of researchers are aware of the occurrence and consequences of water repellency in a wide range of soils, it is still a neglected field in soil science. (Dekker et al., International Turfgrass Society Research Journal, Volume 9, 2001, pages 498-505)
It has been recognized for years that in water repellent soil significant spatial variability can occur both in soil water content and degree of water repellency. Agriculturalists have attacked the soil water repellency problem through the use of wetting agent surfactant compositions. The degree of efficacy among chemistries and formulations has varied significantly. Often, the amount of surfactant required to ameliorate water repellency and/or to enhance infiltration, either perform variably or in an attempt to improve performance, higher rates of wetting agents are applied, such elevated rates often becoming injurious to plants.
Hydrophobic soils can cause problems on golf courses and other turf areas, in nurseries and greenhouses, and in open fields. Golf course managers commonly report problems with localized dry spots on their greens. These dry spots become a serious turf management problem during the summer months, especially during periods of drought. Despite frequent irrigation, the soil in these spots resists wetting, resulting in patches of dead or severely wilted turf. The water applied wets the turf but does not adequately penetrate the soil surface to reach the root zone.
Nursery operators sometimes encounter hard-to-wet media in pots and greenhouse beds. Farmers who work organic soils often complain that the soil wets too slowly, reducing crop productivity. Problems with hydrophobic soils are also commonly associated with citrus production areas, with locations where mine spoils have been deposited, and with burned-over forestland and grassland.
If water cannot readily penetrate and wet the soil, the availability of moisture to plants is reduced, decreasing the germination rate of seeds, the emergence of seedlings, and the survival and productivity of crop plants. Lack of sufficient water in the soil also reduces the availability of essential nutrients to plants, further limiting growth and productivity. In addition, water that cannot penetrate the soil runs off the surface and increases soil erosion. Water repellency often occurs in localized areas. As a result, the soil wets nonuniformly, and dry spots occur.
In hydrophobic soils, the soil particles are apparently coated with substances that repel water, much like wax. In studies of localized dry spots in turf grass, the soil particles were found to be coated with a complex organic, acidic material that appeared to be the mycelium (growth structure) of a fungus.
Nonionic surfactants, or surface active wetting agents, reduce the surface tension of water, allowing the water molecules to spread out. When applied to water repellent soils in high concentrations, surfactants can improve the ability of water from rain or watering to penetrate the soil surface and thus increase the infiltration rate. However, most nonionic surfactants have significant water solubility and thus are rapidly removed by repeated rains or watering. In addition, most nonionic surfactants have one or more hydroxyl end groups that are easily oxidized or attacked by microbial agents, both of which reduce the durability of the treatment.
The prevention of dew formation on grass blades on managed grass and turf surfaces is also often desirable. The water drops present in dew provide needed moisture for the growth of fungal diseases of turf grasses. If the formation of dew is suppressed, the grass blades can dry out more quickly and thus the growth of fungal diseases can be minimized.
In dry periods, turf can be affected by drought stress. This can manifest itself in a number of ways, and in extreme cases the turf may die. Turf grass maintained on light soil, e.g. sand rootzone golf greens and links golf courses, is particularly prone to drought stress as is turf which is grown in generally poor soil conditions. Curiously, drought stress not only occurs in dry conditions, but also in relatively wet seasons due, for example, to rootbreaks, buried materials close to the surface, or through general inefficiency of an irrigation system.
Soils can also suffer drought stress. Thus, on heavy soils, one of the first signs of drought stress is that surface cracks appear on the soil. It will be appreciated that drought stress, in all its various forms, is undesirable and that it would be advantageous to avoid or reduce it.
So-called soil capping, i.e. crusting of the soil surface, can occur due to the pounding action of raindrops on soil. Capping can give rise to various problems, especially in seedbeds on light soils where it can prevent or reduce seedling emergence, thus resulting in a patchy, uneven sward. It would be desirable to be able to avoid soil capping, or at least reduce its effects.
Additionally, in many places water is becoming an ever decreasing resource, as is evidenced by dry rivers, low water tables and frequent restrictions on water usage. Further, in times of water shortage, it is often amenity users of water (e.g. golf courses etc.) where restrictions are enforced. It would, therefore be highly advantageous to be able to treat turf and soil so as generally to improve their water conservation so as to promote efficient use and minimize wastage.
It is also known that water conservation is a major issue in the United States and other countries, as water becomes an increasingly expensive commodity. Turf, particularly managed turf such as that located at golf courses, athletic fields, office parks and similar areas, uses large amounts of water. In past surveys by the Golf Course Superintendents Association of America (GCSAA), respondents indicated that irrigating an eighteen hole golf course in the U.S., having an average area of 77.7 irrigated acres, required an average of 28.5 million gallons of water each year. Of course the survey indicated regional differences in irrigation demand, with the Southwest US requiring 88 million gallons of water per year while the Mid-Atlantic states required 10 million gallons of water on average.
Among other problems faced in the areas of managed turf is localized dry spot caused by water-repellent soil conditions. Although this hydrophobic soil condition has several possible causes, researchers generally agree that the formation of an organic coating on the soil particles caused by the decomposition of plants and/or organisms causes the problem. The condition is characterized by irregular and isolated areas of problematic turf grass on the golf course, in the lawn or in other areas of turf.
The symptoms of localized dry spot are treated with surfactants, or surface-active agents. Some surfactants used to treat the condition are surfactant polymers. A surfactant polymer generally contains large segments or “blocks” of monomer which are hydrophobic in nature, attached to large blocks, which are hydrophilic in nature. Such surfactant polymers are generally referred to as “block copolymers” and give the polymer its surface-active nature. It is generally accepted that the hydrophobic portion of the surfactant molecule is attracted to the water repellent organic coating on the soil, whereas the hydrophilic portion of the surfactant remains readily accessible to water, thus allowing water to move into the soil profile, rather than running off of the surface.
A large number of surfactants are currently being marketed to manage localized dry spots. Such products are often marketed as soil wetters or wetting agents. Wetting agents are materials that increase the area that a droplet of a given volume of spray mixture will cover on a target. The management approach for using soil wetters and wetting agents generally involves direct application of the agents to the localized, problematic area, on an as needed basis, as part of an overall caring program.
In addition to surfactants, super-absorbing cross-linked polymers, including cross-linked polyacrylamides, have been used to treat localized dry spots. As the soil becomes wet, the cross-linked polymer absorbs water and holds it in the soil. Theoretically, the polymer continues to release stored water to the plant long after the soil would normally have dried. These cross-linked polymers can absorb and hold many times their weight in water.
U.S. Pat. No. 6,481,153 and U.S. Pat. No. 6,591,548 and U.S. Pat. No. 6,675,529 disclose soil additive formulations comprising humic acid redistribution (removal) compounds and methods for reducing water repellency within sandy soils by the application of these formulations. The humic acid redistribution compounds contain substituted succinic acid salts, a polycarboxylic acid salt, and a material to reduce the surface tension of a humic acid waxy coating.
U.S. Pat. No. 6,857,225 and U.S. Pat. No. 6,948,276 describe a soil additive formulation for reducing water repellency comprising a multi-branched wetting agent having an “oxygen-containing polyfunctional base compound and at least three surfactant branches attached thereto, wherein each surfactant branch includes both hydrophilic and hydrophobic constituents.” The formulation also includes a secondary compound that actively lowers the surface tension of humic acid waxy coatings from hydrophobic sand particles. The '225 patent is a method for reducing localized dry spot formation by application of the additive formulation.
U.S. Pat. No. 6,948,276 is directed to a multi-branched regenerating wetting agents for treating sandy soils for long-term reduction of water repellency. Certain novel formulations of turf additives that act in such a manner as to permit proper amounts of moisture to contact root systems in order to reduce dry spots within highly managed turf areas and/or lawns. The inventive formulation comprising multi-branched surfactant compounds with both hydrophobic and hydrophilic constituents within each branch attached to an oxygen-containing polyfunctional base compound permits effective moisture penetration through such localized dry spots for sustained grass growth therein. Importantly, such multi-branched wetting agents provide sustained moisture penetration over a sustained period of time since the individual branches of such compounds may become dissociated from its base polyfunctional compound. Since such branches include both hydrophobic and hydrophilic constituents themselves, and thus act as wetting agents, even after degradation of the initial surfactant compound, long-term wetting and moisture penetration, at least, are permitted. Methods of treating sandy soils with such compounds and formulations thereof are also contemplated within this invention.
Thus, there is a continuing search and a long felt need for wetting agent compositions with increased wetting rate that are able to quickly penetrate and infiltrate the water repellent soil. The use of wetting agent compositions with increased wetting rates, in turn, will result in a more effective wetting of the root zone during rain events and/or irrigation applications, thereby, inducing better plant growth and decreased run-off. There is also an ongoing need for hydrophilic treatments for soils that are durable to repeated exposures to water and resist rapid oxidation and microbial attack. The treatment agent must also not harm plant life exposed to it.